1. Field of the Invention
This invention relates generally to filters for removing particulates from particulate laden gas streams and more specifically to a low-cost disposable particulate filter for removing particulates from the exhaust of internal combustion engines.
2. Background of the Invention
Particulate filters have long been used in a wide range of applications where it is necessary to remove particulate matter from gas or liquid streams with high collection efficiencies. Particulate filters are also being used with increasing frequency to remove soot and other particulate matter from the exhaust of internal combustion engines, particularly diesel engines.
As is well known, the exhaust from diesel engines contains numerous components thought to be harmful to humans, such as unburned hydrocarbons, carbon monoxide (CO), oxides of nitrogen (NO.sub.x), sulfur dioxide (SO.sub.2), sulfates, and various aerosols. The solid particulate matter in diesel engine exhaust typically comprises small, solid, irregularly shaped particles, which are themselves agglomerates of smaller sub-particles. The particles may often have high molecular weight hydrocarbons absorbed on their surfaces, thus making the particulate matter a complex mixture of pure carbon and various kinds of organic materials, the sizes of which may range from very small particles of about 0.01 microns to relatively large clusters in the range of 10-30 microns. Turbocharged diesel engines tend to emit more of the smaller particles, but with much lower levels of retained organic compounds. Particle sizes of 10 microns and less are considered to be the most damaging to human lungs, and certain characteristic components of diesel exhaust particulate emissions are known carcinogens.
Many different types of exhaust treatment systems have been developed in an attempt to remove or eliminate the particulate matter before it is released into the atmosphere. Such systems almost always use some type of filter to trap the particles in the exhaust stream. Ceramic materials, stainless steel wire mesh, and other filter materials capable of withstanding the high-temperature exhaust gases have been tried and are being used with some degree of success. Unfortunately, because of the large quantities of particulate matter that are generated by most diesel engines, most filters tend to clog quickly, which increases back pressure in the engine exhaust and adversely affects the performance and efficiency of the engine. Of course, one remedy is to replace the filter when the back pressure exceeds some predetermined limit. However, the metal or ceramic filter materials used in most exhaust filters are expensive, so it is not practical to throw away the filters when they become clogged. As a result, several filter regeneration methods have been developed in attempts to solve the clogging problem. Unfortunately, the space, cost, and energy consumption required by such regeneration methods are substantial. Furthermore, in-situ filter regeneration techniques, where the filters rely on the hot exhaust gases themselves to raise the temperature of the filter high enough to oxidize the trapped particles, cannot be used with engines that operate under light duty-cycles.
Another type of exhaust treatment system, described in my co-pending patent application, Ser. No. 07/765,689, filed on Sep. 26, 1991, now U.S. Pat. No. 5,272,874, represents a significant breakthrough in exhaust filtration technology. That system uses an exhaust-to-water heat exchanger to lower the temperature of the exhaust gases, thereby allowing the use of inexpensive, low-temperature filter materials to trap exhaust-borne particulates.
While the low-temperature particulate filter described in that patent is effective in removing the particulate matter with a high collection efficiency, it is not without its drawbacks. For example, while the low-temperature filter is substantially less expensive then filters made from materials that can withstand higher temperatures, such as ceramic materials, the filter requires metal end plates and metal screens on both sides of the filter media to provide the required structural integrity and support for the filter element. Besides increasing the cost of the filter, such metal components complicate filter disposal. That is, since the filter media traps primarily carbon particles and other unburned hydrocarbons, old filters are well suited for incineration. However, the presence of the metal components complicates the incineration process. The metal components are also a concern if the old filters are disposed of in landfills.
Another disadvantage associated with the filter described in U.S. Pat. No. 5,272,874 is that it is a conventional cylindrical design, which limits the surface area of the filter. Obviously, it is desirable to maximize the surface area of the filter to reduce the exhaust back pressure and to increase filter life. Of course, one way to increase the surface area of such a cylindrical filter would be to increase either the diameter or length of the filter, or both. Another way to increase the filter area would be to install another filter element in parallel with the existing element. Unfortunately, however, neither of the foregoing options are particularly desirable, since the space that can be devoted to the filter assembly is limited in most installations. In fact, the space constraints in most vehicles require that the filter be as small as possible.
Another problem associated with conventional cylindrical filter designs used in such applications is that the filters do not adequately contain the fine, powdery soot accumulated therein, and a substantial amount of the loosely captured soot usually falls out of the filter after it is removed from the filter housing. Besides making a mess, the escaping soot can pose health problems because of the extremely small sizes of the particles released.
Consequently, there remains a need for an improved disposable particulate filter that can be used in conjunction with exhaust treatment systems for internal combustion engines. Such a filter should provide a large surface area to minimize the pressure drop across the filter element and maximize filter life, but without substantially increasing the size of the filter. The filter should also be inexpensive to manufacture, yet maintain high collection efficiency, all while requiring few or no integral metal components.